41 
Northern Hemisphere when the greatest dynamic measurements lie 
- on the right hand. (See p. 35.) 
It is customary to chart the dynamic form of several isobaric 
surfaces of observation—i. e., at several standard decibar depths. 
Such methods permit one to follow the movements in several differ- 
ent layers, each one of which contains supplementary information re- 
garding the combined movements of the entire mass. It is often de- 
sirable to chart the current at a depth below the surface where tempo- 
rary fluctuations, such as those due to variable winds, become elimi- 
nated. For example, if surface conditions are shown on a 2,000- 
decibar chart, then 2,000-100 decibars will reveal the movement 
prevailing at a depth of about 100 meters below the surface. 
FRICTION 
In the resolution of current forces it was shown that the primary 
force, AH), giving rise to a constant current was equally opposed by 
the effect of terrestial rotation; provided there were no accelerating 
force of friction (see fig. 7, p.23). Friction, it was pointed out, was of 
no practical importance to the discussion of gradient currents, but was, 
however, a factor of considerable magnitude in dealing with the 
tangential effect of winds upon surface layers, and thus from a theo- 
retical view, at least, the entire subject merits discussion here. There 
are two kinds of friction, viz, (1) molecular and (2) virtual. Mo- 
lecular friction, or viscosity in the sea, is of insignificant consequence 
and may be completely disregarded. Virtual friction or real fric- 
tion not only considers the viscosity existing between any two layers 
of different velocities, but it also includes the resistance due to tur- 
bulence. Virtual friction exerts itself to a varying degree in the 
sea; for example, a homogeneous water mass mixed by the turbulent 
effect of the winds possesses greater friction dynamically than pre- 
vails in a region of more pronouncedly stratified water. Since we 
consider movements in a horizontal direction only, the friction force 
is consequently confined to that direction. The particular form 
given to the velocity diagram of a current contains direct informa- 
tion bearing upon the degree of friction prevailing at the time of 
observation, and thus wherever constant currents are established 
and flowing in the same direction at different levels, the acceleration 
or retardation of friction acts either in the same or an opposite direc- 
tion to the flow of the water particles. 
If we examine several velocity diagrams we find that there are three 
general types as shown by Figure 20, page 42. In ‘‘A”’ the velocity of 
particle ‘‘a”’ is equal to the mean of the velocities of a water particle, 
or water layer, adjacently above and adjacently below; then the 
accelerating effect of the former is equal to the retarding effect of 
the latter, and the resulting value of friction in such currents is zero. 
This is a form often observed in a voluminous current such as the 
